Ever since antiquity, people have created pictures. In ancient times, pictures were exclusively static pictures generated, for example, by painting, or drawing on a surface. In modern times, photography has provided the ability of creating pictures through technological tools, and cinematography has provided the ability to create moving pictures, first in black and white and, later, in color. More recently, electronic displays, such as computer monitors, TV sets, and projectors, have become the most common devices for displaying moving pictures.
In parallel with the development of electronic displays, the capture and storage of images, moving or not, have also evolved from traditional photography, which used film based on chemical reactions, to electronic devices. Nowadays, movies and still images are captured, stored, transmitted and reproduced as digital signals and data structures in all but a handful of cases.
With very few exceptions, electronic displays generate pictures that are perceived the same regardless of the position of the viewer. Indeed, a lot of engineering effort has been devoted to achieving displays that have a wide viewing angle and exhibit minimal degradation of the picture even for viewers looking at the display from directions that are very different from optimal. There are, however, situations where it is desirable to have a display that shows different pictures when viewed from different angles. Such displays are known as multi-view displays.
Conventional (non-multi-view) electronic displays generate visible images by means of an array of pixels. The term “pixel” is widely used in conjunction with images and image processing. It is a contraction of “picture element” and it refers to the smallest image-forming unit of a display. Generally electronic displays have a large number of pixels; for example, a typical television set of the so-called High-Definition (HD) variety has about two million pixels. High-end television sets such as so-called 4K sets can have more than eight million pixels.
Each pixel of an electronic display emits light of a particular color and brightness, such that the collection of all the pixels forms a pattern that is perceived as an image by the human eye. Typically, the light emitted by a single pixel can be completely specified by providing three numbers, one for each of the three primary colors, red, green, and blue. In typical displays, eight bits are used for specifying each of the three numbers, such that a total of twenty-four bits per pixel are needed in order to fully specify an image to be displayed by an electronic display. High-end display might need more than twenty-four bits per pixel.
At twenty-four bits per pixel, an HD display needs almost fifty million bits to specify a complete image. Moving pictures are realized as a sequence of static images (aka frames) that follow one another at a high rate (the frame rate). A typical frame rate is 60 frames per second, which, at fifty million bits per frame, yields a bit rate of three billion bits per second. This is the bit rate needed to convey a full moving picture to a high-definition television set. More advanced displays such as 4K television sets need proportionally higher bit rates.
Even with modern electronics and telecommunication capabilities, these are high bit rates to handle. Fortunately, many decades of research by a large number of researchers have yielded compression techniques that make it possible to encode full-motion video streams into much reduced bit rates. Compression ratios approaching 1000:1 or even higher have been achieved, and they can support surprisingly good picture quality. It is the use of such compression ratios that has enabled the ubiquity of visual content in modern society.
FIG. 1 is a block diagram that shows an example of how a conventional (non-multi-view) electronic display might be used. The diagram comprises a source of visual content 110. The term “visual content” should be understood to refer to any type of material that can be represented as an image or images, whether moving or static, suitable for visual consumption by a human viewer. In the example of FIG. 1, the source of visual content is a movie database that stores a plurality of movies. One of the movies is being retrieved from the database and conveyed to electronic display 130.
In the example of FIG. 1, the electronic display 130 might be a movie projector in a movie theatre. The visual content representing the movie is conveyed to the site of the projector as visual content stream 115. The medium that supports the visual content stream is not explicitly identified in the figure, but those skilled in the art will know that a variety of possibilities exist. For example, the medium might be a so-called Ethernet cable.
As noted above, the availability of compression techniques makes it possible for the visual content to be carried by a bit rate that is much less than the bit rate needed to fully specify the images to be displayed by the electronic display 130. However, the compressed visual content stream needs to be converted into such fully-specified images for the electronic display to be able to properly activate all its pixels such that each pixel emits light of the desired color and brightness. In particular, if each pixel requires twenty-four bits for specifying the light to be emitted, the visual content stream needs to be converted into a plurality of 24-bit data elements, one for each pixel, at the frame rate. In FIG. 1, this task is performed by the graphic processing unit (GPU) 120, which is connected to the electronic display via connection 125. As noted above, if the electronic display is of the HD variety, about 3 billion bits per second need to be transferred over connection 125. In a movie theater, the projector is likely to be better than a plain HD projector, in which case the bit rate carried by connection 125 is expected to be higher.
FIG. 2 illustrates the principle of operation of a typical image projector. The illustration applies to old-fashioned movie projectors and slide projectors that project images from film, and it also applies to modern electronic projectors. In all such cases, the image to be projected onto a screen originates as a bright image that emits light, shown in the figure as bright image 210. In the case where film is used for the image, the light comes from a bright light bulb behind the film, and the film acts as a filter that selectively allows the passage of light of different colors and brightness in different portions of the image. A similar technique is used in some modern projectors wherein the filter might be a liquid-Crystal Display (LCD) module or some other type of electronic light filter, instead of film. Alternatively, the bright image might be generated by an array of bright sources such as, for example, light-emitting diodes (LED), or by digital micromirror devices that reflect light from a separate source.
In a modern electronic projector, the bright image is generated as a collection of pixels, wherein each pixel emits light of a particular color and brightness in a wide range of directions. In a projector, as depicted in FIG. 2, some of the light emitted by each pixel is collected by a lens 220. In the figure, two pixels are highlighted explicitly as pixel 230-1 and 230-2. The figure shows, for example, the light 240-1 emitted by pixel 230-1 and collected by the lens 220. The lens is adjusted such that the light collected from the pixel is focused into a light beam 250-1 focused on a projection screen some distance away (the screen is not shown explicitly in the figure). When the light beam 250-1 reaches the screen, it produces a bright spot on the screen. The color and brightness of the spot are the same as the color and brightness of pixel 230-1 in the bright image 210. The light 240-2 from pixel 230-2 is also processed by the lens 220 in similar fashion, such that it also produces a bright spot on the screen whose color and brightness are the same as the color and brightness of pixel 230-2. All the pixels of the bright image 210 produce bright spots on the screen in similar fashion. The collection of all the bright spots on the screen forms the projected image.
FIG. 3 is a block diagram that shows an example of a system for distributing data to a plurality of electronic displays. In the example, the electronic displays are projectors. This might be applicable, for example, to a multiplex movie theater with a plurality of viewing rooms. The diagram comprises a source of visual content 310 which, in the example, is a movie database that stores a plurality of movies.
The source of visual content generates a plurality of visual content streams, some of which are depicted in the block diagram as visual content streams 315-1, 315-2, and 315-3. They might be, for example, different movies to be shown in different rooms of the multiplex movie theater; although it's also possible that some rooms might be showing the same movie, in which case two or more of the visual content streams would carry the same movie, possibly with different starting times.
As in FIG. 1, each visual content stream is received by a GPU. Three GPUs are depicted in the figure as 320-1, 320-2, and 320-3. Each GPU converts the corresponding visual content stream into fully-specified images for the associated electronic display. The three GPUs shown in the figure are associated with and connected to three electronic displays 330-1, 330-2, and 330-3 through connections 325-1, 325-2, and 325-3, respectively.
The system of FIG. 3 is practically realizable in the prior art because, in the example of a multiplex movie theater, the number of electronic displays (projectors) is not too large. Few movie theaters have more than 10-20 viewing rooms. Other situations exist wherein a larger number of electronic displays need to receive independent visual content streams. For example, in a modern airplane, it often happens that each passenger has an individual display for viewing movies or other visual content in a personalized fashion. In such a situation, the block diagram of FIG. 3 might be applicable, with the number of electronic displays being as large as possibly a few hundred. Even with such larger numbers of displays, such systems are practically realizable, especially because, in an airplane, each electronic display is likely to have much less resolution than a projector in a movie theater.
In FIG. 3, the visual content streams are depicted as being distributed as individual links emanating from the source of visual content. Such a structure is known as a “star topology”. Those skilled in the art will know that other topologies are also possible for a system that distributes data to a plurality of destinations; for example, such a data distribution system might be realized as a data bus. Other topologies are also possible and well known in the art.
As hinted above, block diagrams such as in FIG. 3 are practically realizable because of the moderate volume of data to be distributed to the GPUs. Data distribution systems are available that provide the necessary capacity. However, if the number of electronic displays becomes very large (for example, hundreds of thousands or millions) the volume of data to be distributed becomes so large as to exceed the capabilities of available data distribution systems. This is likely to be especially true if the electronic displays have a large number of pixels.